You could just copy/paste the formula field from one document (e.g. the one you already have) to the other...

Creating one from scratch entails pressing Ctrl-F9 to create a pair of field braces, filling them with whatever text you need, adding another pair of field braces (alongside or nested, according to the formula requirements), filling them with whatever text you need, and so on, till you've built up the entire formula. When doing it manually, you need to ensure there are spaces between the various expressions (otherwise IF tests, for example, won't work) and to get the structure & logic right. The tutorial you now have contains many examples.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]